The present invention relates to a drive control apparatus for a brushless motor that performs duty ratio control of the brushless motor by modulating the pulse width of a drive pulse in correspondence to the setting for the rotation rate.
Methods for controlling the speed of a brushless motor include the one disclosed in Japanese Unexamined Patent Publication No. H 5-34781, which implements speed control by changing the pulse width of a drive pulse supplied to the exciting coils in order to apply a rotating magnetic field to the rotor through pulse-width modulation (PWM), for instance. In this pulse-width modulation a triangular wave having a specific frequency and a threshold value corresponding to the rotation rate that has been set are compared with each other, and if the rotation rate setting is smaller, the pulse width (duty ratio) of the drive pulse is reduced to lower the rotating speed of the rotor, whereas if the rotation rate setting is larger, the pulse width (duty ratio) of the drive pulse is increased to raise the rotating speed of the rotor.
However, the structure described above has a problem in that, when the brushless motor is being driven at a given rotation rate setting, if the source voltage fluctuates for any reason, the actual rotation rate, too, fluctuates as a result. For instance, if the source voltage rises, the peak voltage of the drive pulse becomes higher even if there is no change in the duty ratio, and the rotation becomes faster due to an increase in the current supplied to the exciting coils. In contrast, if source voltage falls, since the peak voltage of the drive pulse becomes lower, the rotation slows down due to a reduction in the current supplied to the exciting coils.
In the low speed rotation range and the medium speed rotation range, such fluctuations in the source voltage will only result in less stable drive of the motor, but when the motor is rotating at high speed, if the source voltage shifts upward, an excess voltage is applied which may result in melting of the fuse or damage to the motor.